Originally Posted by Wing Gundam Zero
Budokai 3 Uub 15
You cleared Dragon Universe in Dragon Ball Z: Budokai 3 with Uub
You gain Uub by killing Kid Buu in Goku's DU by a spirit bomb in his normal state. Your ultimate attack needs to be a success for the right cutscene to unlock.
NOTE: This can only be done in your second playthrough of DU
I've just unlocked Uub on my 1st playthrough with Goku, so I think you can update the guide sayin it's not mandatory to be in the second one.